1. Field of the Invention
The present invention relates generally to a liquid-crystal display (LCD) device. More particularly, the invention relates to a LCD device of the type having a first substrate on which switching elements are formed, a second substrate on which a color layer is formed, and a liquid-crystal layer located between the first and second substrates; and a method of evaluating the LCD device, where the second substrate includes no electrodes formed at nearer positions to the liquid-crystal layer with respect to the color layer. A known structure of this type is the “In-Plane Switching (IPS) type LCD device.
2. Description of the Related Art
Active-matrix addressing LCD devices, which use Thin-Film Transistors (TFTs) as switching elements for pixels and which are capable of providing high-grade image quality, have been extensively used as monitoring devices (or, monitors) for space-saving desktop computers or the like.
As the operation mode of active-matrix addressing LCD devices, two modes, i.e., the Twisted Nematic (TN) mode and the IPS mode, have been already known. With the TN mode, initially aligned liquid-crystal molecules in the liquid-crystal layer are rotated around a direction perpendicular to the first and second transparent substrates on operation. Unlike this, with the IPS mode, initially aligned liquid-crystal molecules in the liquid-crystal layer are rotated in a plane approximately parallel to the first and second transparent substrates on operation.
With the IPS mode LCD device, pixel electrodes and common electrodes are alternately formed on the TFT substrate in such a way as to form comb-teeth in the respective pixel areas. A voltage is applied across the pixel electrodes and the common electrodes to generate desired electric field approximately parallel to the TFT substrate in the liquid-crystal layer, thereby changing the alignment direction of the liquid-crystal molecules in the desired pixel areas. In this way, the amount of the light penetrating through the liquid-crystal layer is controlled as desired, generating images on the screen of the device. As seen from this explanation, the liquid-crystal molecules are rotated or twisted in an approximately parallel plane to the TFT substrate and thus, there is a feature that good images are displayable within a wider viewing angle compared with the TN mode LCD device.
A prior-art structure of the IPS-type LCD device is explained below with reference to FIG. 1.
As shown in FIG. 1, the prior-art IPS-type LCD device comprises a first substrate (i.e., a Thin-Film Transistor (TFT) substrate) S101, a second substrate (i.e., a color filter (CF) substrate) S102 located parallel to the substrate S101, and a liquid-crystal layer 110 sandwiched by these substrates S101 and S102.
The TFT substrate S101 comprises a first transparent plate 101, an interlayer dielectric layer 104 formed on the inner surface of the plate 101, a passivation layer 108 formed on the layer 104, and an alignment layer 109a formed on the layer 108. Scanning lines (not shown) are formed in parallel to each other on the plate 101. Signal lines (not shown) are formed in parallel to each other on the layer 104. The scanning lines and the signal lines are perpendicular to each other, forming pixel areas at their intersections. TFTs (not shown) are formed at the respective pixel areas (i.e., the respective intersections of the scanning lines and the signal lines) in a matrix array. In each of the pixel areas, a pixel electrode 107 and a common electrode 103 are formed in such a way as to form comb teeth. All the common electrodes 103 arranged at the respective pixels are electrically connected to each other by way of common electrode lines (not shown), to which a common electric potential is applied on operation.
The CF substrate S102 comprises a second transparent plate 111. A patterned black matrix 112 for preventing unwanted light from penetrating through the substrate S102 and a patterned color layer 113 for displaying color images are formed on the inner surface of the plate 111. The matrix 112 and the layer 113 cover almost all the inner surface of the plate 111. The layer 113 includes sublayers for red (R), green (g), and blue (B) colors. An overcoat layer 114 is formed on the black matrix 112 and the color layer 113 to protect the same. The combination of the black matrix 112, the color layer 113, and the overcoat layer 114 is termed the color filter (CF). An alignment layer 109b is formed on the layer 114. On the other hand, a conductive layer 115 is formed on the outer surface of the plate 111.
The alignment layers 109a and 109b, which are opposite to each other by way of the liquid-crystal layer 110, are used to align homogeneously the liquid-crystal molecules in the layer 110 in a predetermined direction (i.e., the initial alignment direction), which has a specified angle with respect to the longitudinal axis of the pixel electrodes 107.
A polarizer plate 116a is attached to the outer surface of the first plate 101. A polarizer plate 116b is attached to the outer surface of the conductive layer 115. The polarization axis of the plate 116a is perpendicular to that of the plate 116b. One of the polarization axes of the plates 116a and 116b is set to be parallel to the above-described initial alignment direction of the liquid-crystal molecules.
On operation, electric potential is selectively applied to the pixel electrodes 107 as necessary by way of the TFTs, thereby forming lateral electric field between the pixel electrodes 107 and the corresponding common electrodes 103. Thus, in the selected pixels, the liquid-crystal molecules are rotated (in other words, the molecules are twisted) in a plane parallel to the substrates S101 and S102, thereby displaying desired images on the screen.
The reference numeral 117 shown in FIG. 1 denotes an electric line of force generated by the voltage between to the common electrode 103 and the pixel electrode 107 in each pixel.
The LCD device of this type has ever been used mainly for monitors for notebook-type and desktop-type personal computers. However, recently, it has become used in other fields such as television (TV) and multimedia. Thus, there is the need to not only to improve the viewing angle characteristics but also to widen the chromaticity. With the visual instruments (e.g., TV monitors) designed for the TV field, the transmission method of picture signals including the hue has already been standardized. A typical one of the standards relating to the picture signal transmission is “NTSC” (National Television System Committee) employed in the U.S.A. and Japan. Another is “EBU” (European Broadcasting Union) employed in European countries. To expand the application of LCD devices to the TV and multimedia fields in the future, it is necessary to fabricate LCD devices in such a way as to meet both of the standards, NTSC and EBU.
Conventionally, LCD devices have been fabricated to satisfy the NTSC standard that requires the devices to have a chromaticity region of approximately 60%. On the other hand, to satisfy the EBU standard that requires the devices to have a chromaticity region of approximately 70%, constituting elements or parts of a LCD device have to be improved. In particular, the optical characteristic of the color filter has to be improved. To cope with the improvement required, the kind and combination/arrangement of a pigment or pigments used for the color filter needs to be adjusted or coordinated.
On the other hand, various display defects or failure caused by the optical characteristic of constituting elements and/or the characteristic of liquid crystal have been known. One of the defects is termed the “white-color irregularity”, which is caused by the color filter. The “white-color irregularity” is a phenomenon that when voltage is applied to all the pixel electrodes to thereby display a black image on the entire screen, irregularity or unevenness of darkness is observed on the screen. This is because the transmittance is kept unequal to zero at a part of the pixels even if voltage is applied to all the pixel electrodes. The cause of the white-color irregularity is thought that unwanted electric current flows through ionic substance existing in the liquid crystal, thereby lowering the voltage applied across the pixel electrode and the corresponding common electrode in the part of the pixels, although the voltage applied across the pixel electrode and the corresponding common electrode should be kept constant.
To suppress or prevent the white-color irregularity, some measures have been developed and disclosed. Examples of the measures are disclosed in the Japanese Non-Examined Patent Publication Nos. 2001-305332 published in Oct. 31, 2001 and 2000-186225 published in Jul. 4, 2000, in which the “white-color irregularity” is suppressed by directing the attention to impurities contained in the color filter.
The measure disclosed in the Publication No. 2000-186225 is a method of producing a pigment for a color filter, in which fuming sulfuric acid or sulfuric acid is used as a solvent for chlorination and/or bromination of a compound having a copper phthalocyanine skeleton. In this method, the ionic impurities, which are likely to dissociate and to apply bad effects to the performance of a LCD device, are reduced. As a result, the “white-color irregularity” can be suppressed.
The measure disclosed in the Publication No. 2001-305332 is a resin composition for a color filter, which contains a volatile component and a non-volatile component and which has a specific voltage-sustaining rate of a liquid crystal from which impurities have been extracted. This measure was developed on the following basis.
A resin member forming a color filter, which is contacted with a liquid crystal layer, was regarded as a source of ionic substances. Then, as a characteristic or characteristics having a correlation with the display defect caused by the ionic substances moved from the resin member to the liquid-crystal layer, attentions were paid to the voltage-sustaining rate of the liquid crystal and the remaining DC voltage (ΔE), where impurities were extracted from the components of the resin composition of the resin member.
With the above-described two measures, ionic substances containing in the color filter are reduced and therefore, the “while-color irregularity” can be suppressed. However, there is another display defect termed the “red-color irregularity”. This defect is caused by the fact that the light penetrating through the green sublayers (which contain a green pigment) of the color layer is reduced and thus, the whole displayed image on the screen is observed reddish irregularly. Since the “red-color irregularity” is not caused by ionic substances existing in the color filter, this defect is unable to be prevented or eliminated by the above-described known measures.